1. Field of the Invention
This invention generally relates to classifiers used for separating relatively fine particles from a particulate material, such as sand used in the road construction industry. More particularly, this invention is directed to an air classifier having a compact construction with reduced energy requirements.
2. Description of the Prior Art
Sand used in the construction of asphalt and concrete roads is sized to provide an appropriate particle size distribution which excludes extremely fine particles (dust) that are undesirable for the final asphalt or concrete mix. Removal of the fine particles is often performed after the sand has been processed through a crusher and then sized. Typical methods for removing fine particles from sand and other particulate matter include placing the sand on a wash screen, and then flowing water over the screen to carry finer particles, e.g., -200 mesh (less than 75 .mu.m) through the screen. The resulting sand slurry is then pumped to a settling pond. The sand remaining on the screen typically has a moisture content of about 8%, and therefore requires drying before being added to an aggregate mixture for use in the production of asphalt or concrete.
Though widely used, wet screening processes have several disadvantages. The use of water can pose a problem if the construction site is in an arid area, and the necessity to dry the processed sand adds undesirable equipment and fuel costs. The environmental impact of settling ponds is also undesirable. Therefore, what is needed is a process and apparatus for removing fine particles from sand that is suitable for use in such applications as asphalt and concrete road construction. In addition to eliminating the requirement for water, a suitable apparatus would also preferably have minimal energy requirements and provide a compact operating unit for ease of transportation.